A Violent History of Time

January
24, 2008: From mother Earth, the night sky can look
peaceful and unchanging, but the universe as seen in gamma-rays
is a place of sudden and chaotic violence. Using gamma-ray
telescopes, astronomers witness short but tremendously intense
explosions called gamma-ray bursts, and there is nothing more
powerful.

No
one is sure what causes gamma-ray bursts. Favored possibilities
include the collision of two neutron stars or a sort of super-supernova
that occurs when extremely massive stars explode. One thing
is certain: gamma-ray bursts happen in galaxies far, far away
-- so far away that the distances are called "cosmological,"
beyond ordinary comprehension.

Think
about this: When you look up at the night sky, you are looking
at the ultimate history book – one that goes back to the very
beginning of what we call time. And each star is a chapter
in the book. You are not really seeing the stars as they are
now. You are looking at stars as they used to be when their
light left them long ago. And the deeper we peer into space,
the farther back in time we are looking. In fact, light from
the galaxies farthest away is billions of years old.

"Gamma-ray
bursts are so bright we can see them from billions of light
years away, which means they occurred billions of years ago,
and we see them as they looked then," says Charles Meegan
of NASA's Marshall Space Flight Center. "They can help
us look back in time and teach us something about the conditions
in the early universe. In gamma-ray bursts, we may be seeing
the first generation of stars, from the earliest galaxies
created after the Big Bang."

Not
only do gamma-ray bursts help scientists learn about our universe's
history; they also help explain its physics. But the tricky
part in studying a gamma-ray burst is catching it before it
disappears. Each burst happens and fades so fast that it's
hard to detect them all. It's like trying to capture every
single firefly's flash on a summer night with an ordinary
camera.

NASA's
Gamma-Ray Large Area Space Telescope, GLAST for short, will
soon help in the chase. More on that in a minute, but first,
let us set the stage with a little history.

Scientists
have been hot on the gamma-ray trail for years, but the bursts
were actually discovered by accident. During the Cold War
in the 1960s, US satellites keeping an eye out for Soviet
nuclear testing in violation of the Limited Test Ban Treaty
detected intense bursts of gamma radiation. But the bursts
weren't coming from the Soviet Union. Scientists realized
that the bursts were coming from space!

Left:
Gamma-ray bursts light up the sky like cosmic flashbulbs.
The trace on the right is called the "light curve"
of the burst.

Quickly,
gamma-rays bursts became one of the most compelling mysteries
of astronomy, and NASA decided to build a Great Observatory
to map the gamma-ray sky. In the 1990s, the Compton Gamma-ray
Observatory discovered more than 400 new gamma-ray sources
and recorded 2704 gamma-ray bursts, detailing the gamma-ray
universe early satellites had merely glimpsed. Most importantly,
Compton uncovered evidence that gamma-ray bursts issued not
from the Milky Way, but from staggeringly distant galaxies.

To
be seen at such distances, the explosions had to be almost
impossibly violent, astronomers realized. In a way, this was
no surprise. Gamma rays are by their very nature a herald
of great energy and violence. Consider the following: gamma
rays are a super-energetic form of light. Ordinary visible
photons, the kind we see with the human eye, have energies
of about 2 to 3 electron-volts. Gamma-ray photons have energies
greater than 10 giga-electron-volts (GeV), billions of times
that of ordinary light. Ground-based observatories have detected
gamma-rays of even higher energy – thousands of GeV.

In
May 2008, NASA will launch GLAST to welcome these high-energy
messengers. GLAST's main instrument, the Large Area Telescope
(LAT), will make pioneering observations of gamma-ray bursts
at higher energies than ever before from space. It is expected
to accurately locate 50 or so bursts per year. Meanwhile,
another instrument onboard GLAST, the GLAST Burst Monitor
(GBM), will monitor gamma-ray bursts at lower energies.

Right:
Bristling with detectors, GLAST awaits launch in a General
Dynamics clean room. Click to view the entire
observatory.

By
working together, these two instruments will capture the whole
energy range of these cosmic fireflies – from 10 thousand
eV to 100 giga-electron-volts.

"Capturing
the events in more than one wavelength will help scientists
understand more about them, kind of like seeing in color instead
of black and white," says Meegan. "We can't reproduce
in any laboratory the extreme physical conditions that occur
in gamma-ray bursts, so we don't understand how they work.
By studying them with these instruments, we may learn some
new physics about matter."

"I
think it is likely that LAT and GBM will see something new
and unpredicted from gamma-ray bursts. They will likely answer
some old questions and raise new ones."

That's
what science always seems to do. Stand by for launch in May
2008!

NASA's
GLAST mission is an astrophysics and particle physics
partnership, developed in collaboration with the U.S.
Department of Energy, along with important contributions
from academic institutions and partners in France, Germany,
Italy, Japan, Sweden, and the U.S.

The
GLAST Burst Monitor is collaboration among scientists
at the Marshall Space Flight Center, the University
of Alabama in Huntsville, the Max Planck Institute for
Extraterrestrial Physics in Germany, and the Los Alamos
National Laboratory. The Principal Investigator is Dr.
Charles Meegan at Marshall Space Flight Center. Dr.
Jochen Greiner at Max Planck Institute for Extraterrestrial
Physics is co-Principal Investigator.